Materials used in coatings and thin films as electrical sliding contacts require stable low electrical contact resistance (ECR), as well as acceptable friction and wear performance. Gold is well suited as an electrical contact coating due to its low resistivity and excellent corrosion and oxide resistance and has found widespread industrial use due to its ease of deposition via electroplating. See P. Goodman, Gold Bulletin 35, 21 (2002). However, the high ductility and low yield strength of pure Au lend to poor tribological performance in sliding electrical contacts. Unlubricated sliding Au-on-Au contacts tend to exhibit unacceptable amounts of adhesive wear and friction coefficients exceeding 1.0 as a result of high real to apparent contact area. See F. P. Bowden and D. Tabor, The friction and lubrication of solids, Vol. 1 (Oxford university press), 2001; and M. Antler, IEEE Transactions on Components Hybrids and Manufacturing Technology 4, 15 (1981). However an increase in real area of contact between the contact pair minimizes the contribution of constriction resistance to the ECR of the system. See R. Holm and E. Holm, Electric contacts: theory and application (Springer-Verlag New York), 1967. The balance of these mechanisms in metallic friction and wear with ECR is highly dependent on the real area of contact and thus the mechanical properties of the Au film.
The most common approach to improving the tribological performance of Au films is to increase the film hardness by alloying with minute amounts of transition metals in electroplating, primarily Co and Ni, known as hard Au. See P. Goodman, Gold Bulletin 35, 21 (2002), and M. Antler, Thin Solid Films 84, 245 (1981). The primary mechanism for the increase in hardness is attributed to Hall-Petch strengthening achieved via grain refinement by boundary pinning during codeposition. See C. C. Lo et al., Journal of Applied Physics 50, 6887 (1979). However, the introduction of non-noble transition metals can result in surface oxide film formation via solute diffusion and increase ECR by an order of magnitude or greater with only a few monolayers. See M. Antler, Plating and Surface Finishing 85, 85 (1998); H. G. Tompkins, Journal of The Electrochemical Society 122, 983 (1975); and H. G. Tompkins and M. R. Pinnel, Journal of Applied Physics 48, 3144 (1977).
Ion beam modification (IBM) is a well-established technique for tailoring the electrical, thermal, optical, and mechanical properties of materials. This wide range of material properties that can be tailored by ion beams are a result of the unique non-equilibrium microstructures that can be created by IBM. See M. Nastasi, M., Ion-solid interactions: fundamentals and applications (Cambridge University Press) 1996; and S. Zinkle, Radiation effects and defects in solids 148, 447 (1999). In ductile metals, it has previously been shown that IBM can significantly increase the hardness and strength of the film often at the deterioration of ductility. See J. A. Knapp et al., Journal of Applied Physics 103, 013518 (2008); G. S. Was, Progress in Surface Science 32, 211 (1989); and J. Sharon et al., Materials Research Letters, 1 (2013). However reports on the use of IBM to improve tribological performance of metals has mainly focused on steel and Ti alloys implanted with metallic and other ion species such as N that can form precipitates or react chemically with host species to form new phases. See N. Hartley, Thin Solid Films 64, 177 (1979); P. Sioshansi, Thin Solid Films 118, 61 (1984); M. Iwaki, Materials Science and Engineering 90, 263 (1987); and J. Onate et al., Thin Solid Films 317, 471 (1998). There is a single report describing ion implantation in thin metallic films of Cu, Au, and Al that resulted in an increase in hardness. However, improved wear was only observed in Cu. Further, there was no report of the effect of ion implantation on friction or electrical contact resistance. See J. Y. Robic et al., Nuclear Instruments & Methods 182, 919 (1981).
Therefore, a need remains for noble metal electrical contact coatings that can achieve suitable friction and wear behavior with inherently stable low ECR.